1. Field of the Invention
The invention relates to a focusing system for charged particles, an electron microscopy system and an electron microscopy method.
2. Brief-Description of Related Art
In a conventional electron microscopy system and method, a primary electron beam is directed onto a sample to be inspected. The electrons impinging on the sample react with the sample and cause emanance of electrons from the sample. These electrons emanating from the sample are then detected. According to the characteristics of the sample at the position at which the primary electron beam impinges thereupon, more or less electrons will, at constant primary electron beam intensity, emanate from the sample. From an examination of the intensity of the electrons emanating from the sample in dependence of the location, at which the primary electron beam impinges on the sample, structural information regarding the sample or electron microscopic images, respectively, may be obtained.
To this end, the electrons emanating from the sample are, for example, formed to an electron beam by an objective lens of the electron microscopy system. A portion of this beam or the full beam is supplied to an electron detector. In contrast to the primary electron beam, this beam formed of electrons emanating from the sample will be termed secondary electron beam in the context of this application.
The electrons emanating from the sample are generated by the electrons of the primary electron beam through different physical effects. These effects comprise:                Generation of back scattering electrons, which according to a common definition have an energy of more than 50 eV and are abbreviated BSE;        Generation of electrons which according to the common definition have an energy of less than 50 eV and are termed secondary electrons in the narrower sense. These are again discriminated into secondary electrons abbreviated SE1, which are generated near the surface of the sample by an impinging primary electron, and secondary electrons abbreviated SE2 which are for example generated by back scattering electrons emanating from the sample near the sample's surface;        Generation of electrons of the primary electron beam, which do not quite reach the surface of the sample but are reflected just before the sample's surface due to a charging of the sample and which are commonly referred to as mirror electrons; and        Generation of transmission electrons, which are primary electrons traversing the sample and scattered primary electrons and secondary electrons emanating from the sample in a direction of the primary electron beam.        
Independently from the generating mechanism of the electrons emanating from the sample, these are termed secondary electrons in the context of this application, as long as their generation is caused by one or more impinging primary electrons.
For example from U.S. Pat. No. 4,464,571, an electron microscopy system is known, in which the electrons emanating from the sample are examined with respect to their kinetic energy, so as to draw conclusions concerning the generating mechanism of the electrons emanating from the sample. By energy-selectively examining intensities of the secondary electrons having energies within predetermined intervals, it is possible to obtain additional structural information about the examined sample.
With this background, it is an object of the present invention to suggest an electron microscopy system and an electron microscopy method, with which it is possible to obtain structural information in an alternative manner.
Furthermore, an electron microscopy system comprises a focusing system for the charged particles, namely the primary electrons. The focusing system usually comprises a magnetic focusing lens or an electrostatic focusing lens or a combination from a magnetic focusing lens and an electrostatic focusing lens, to focus the charged particles in a focus as small as possible. However, the focusing lens typically has a so-called spherical aberration, such that charged particles traversing the lens in radially outer regions are deflected too much compared with charged particles traversing the lens in radially inner regions, so that the charged particles are not focused in an ideal point, but in a radially extending disk. Further, such focusing lens also has a chromatic aberration, which is caused by particles having a smaller kinetic energy being deflected more than particles with a higher kinetic energy. When focusing charged particles having kinetic energies in an energy band of a non-negligible width, the particles also cannot be focused in an ideal point but only in a radially extending disk.
In the article “Computer Design of High Frequency Electron-Optical Systems” by Laurence C. Oldfield in “Image Processing and Computer-aided Design”, Electron Optics, P. W. Hawkes (ed.), Academic Press, London, pages 370-399, 1973, it is proposed to integrate into a focusing lens a cavity resonator in order to compensate for aberrations of the focusing lens. However, it has turned out that this proposal is insufficient for providing a satisfactory compensation of the aberrations of the focusing lens.